1. Field of the Invention
The invention relates to a power semiconductor module consisting of a ceramic substrate with metal coating on both sides, with at least one semiconductor component, with connections required for contact, and packaging.
2. Description of the Related Art
In comparison with discrete power switches (such as sliced cells, T0220) power semiconductor modules with or without base plate offer the great advantage of internal isolation from a heat sink. This internal isolation is achieved by using ceramic substrates with metal coatings on both sides, which combine both high isolation strength with great thermal conductivity. This design allows efficient design of power circuits since they provide not only base isolation (isolation from the environment), but also functional isolation (the isolation of various regions of a circuit on which components are mounted).
The definitions of the technical terms used in this related art may be found in Chapter 1 in Kxc3x6nig, Rao: xe2x80x9cTeilentladungen in Betriebsmitteln der Energietechnikxe2x80x9d [Partial Discharge in Power Technology Facilities], published by VDE Verlag, 1992, ISBN 3-8007-1764-6.
Power semiconductor modules, which are the starting point of this invention, are sufficiently known.
Germain registration DE 196 51 632 discloses a power semiconductor module with ceramic substrate and a base plate. Both EP 0 750 345 and DE 197 00 963 disclose power semiconductor modules with ceramic substrates without base plates. DE 43 10 466 discloses pressure-bonded configurations with ceramic substrates. It is additionally understood from disclosures in U.S. Pat. No. 5,466,969, that additional components, such as sensors and/or drive circuits, may be integrated in a power module.
These developments in prior-art power semiconductors all have in common the use of a ceramic substrate with metal coatings on both sides. This ceramic substrate is produced by a spinel bond between aluminum oxide (Al2O3) and copper oxide according to a xe2x80x9cdirect copper bondingxe2x80x9d (DCB) process, as described in EP 0 627 760, or according to an active metal brazing (AMB) process.
In addition to copper, it is also conceivable to use aluminum or silver metallization in a similar manner. Methods are also being developed in which an aluminum layer, with aluminum nitrite (AIN) as a ceramic material, is applied to the ceramic material by means of a sintering process. Subsequently, a further metal layer, for example copper, may also be deposited on the aluminum layer.
It is also typical for this type of power semiconductor modules to be filled with a material such as a monomer of silicone rubber that is polymerized after degasification. This silicone rubber ensures isolation.
In all known configurations of these substrates, the surface with the metal coating is smaller than the ceramic surface, which leaves a non-coated surface at the edges of the substrates. Typically this width, and thus the distance from the edge of the metal coating to the edge of the ceramic surface, is the same as the first surface holding the components, which faces away from the heat sink or the base plate and the is same as the second surface which faces the heat sink or the base plate.
Alternatively, the edge of the coating of the second surface is closer to the edge of the ceramic than that of the first surface. This difference is due to the fact that in pressure-bounded power semiconductor modules, for example, where the prime object of the development is a good thermal contact with a heat sink, pressure forces are acting upon the substrate on the peripheral region. Under these circumstances, to prevent the ceramic from breaking, the second coating is applied so that it comes close to the edge of the ceramic. As disclosed in U.S. Pat. No. 5,466,959, additional drive circuits are positionable on the first surface of the substrate. In this case, the metal coating may be left out below the surface used for the drive, to achieve a reduced capacitive coupling.
Generally, requirements for the isolation strength of base isolation are much higher than for the isolation strength of functional isolation. Thus, IEC 1287 requires the following test voltage for base isolation:                               U                      iso            ,            rms                          =                                            2              ·                              U                m                                                    2                                +                      1000            ⁢                          xe2x80x83                        ⁢            V                                              (        I        )            
where Um represents the maximum constantly recurring voltage in the circuit. Voltage Uiso,rme must be applied for one minute during the test of the component (power semiconductor module). The isolation quality of the base isolation depends on how the peripheral region of the ceramic is configured.
In prior art, the peripheral region of the substrate is configured so that the width of the surface between the edge of the metal coating of the first surface and the edge of the ceramic is the same or larger than the width of the surface between the edge of the metal coating of the second surfaces and the edge of the ceramic.
As disclosed in U.S. Pat. No. 5,466,969, the peripheral region of the power component of the circuit arrangement has this type of configuration. Unfortunately, this configuration has disadvantages for the isolation quality of the substrate""s base isolation.
In this type of device, the metal coatings of the two ceramic surfaces act as a plate condensator with the ceramic as a dielectric between the plates. Typically, the metal coating lies on a base plate or a heat sink and thus at a defined reference potential.
The different parts of the metal coating of the first surface may have different potentials. In related art, the metal coatings for optional drive circuits or sensors, etc. often have ground potential. When the power semiconductor module on a normally metallic heat sink is arranged at reference potential that does not necessarily correspond to the ground potential, the result is a very inhomogeneous field path for the electric field in the outer region of the plate condensator.
A high density of equipolar lines represents a high field strength. The isolation quality of the base isolation of the non-coated peripheral region of the ceramic or the region between a metal coating of the first surface which lies between a high potential and another potential, is determined by the field strength directly on the surface, tangentially to the first surface. This field strength is represented by the tangential distance of the equipotential lines.
The highest density of equipotential lines on the ceramic surface, and thus the most critical region for the isolation strength of the base isolation, is on the first surface of the ceramic immediately following the surface of the metal coating with the high potential.
The configuration of the peripheral region of a substrate has a decisive effect on the isolation and partial-discharge strength of power semiconductor modules. This effect is less significant in the case of thinner ceramic layers. With a ceramic thickness of 0.38 mm, (which is customary today), this effect becomes dominant above 5000 V.
With the integration of the above mentioned additional functions into the power semiconductor module, the isolation of these components, which are directly on the substrate, becomes another important characteristic of reliability.
Since, for example, the sensor signals are being evaluated directly by the drive circuits, an electrical separation between the power circuit and the sensor must be ensured for proper operation. This electrical separation is achieved by means of an arrangement of two metal coatings isolated from each other. Within the power circuit, only a functional isolation must be ensured for metal coatings thus isolated from each other. On the other hand, a base isolation must be established for the additional integrated functions (such as sensors).
In general, the requirements for base isolation are higher, depending on the particular application. For example, in the series-connection of power semiconductor modules, the reference potential of an individual power semiconductor module dos not have to be identical to the ground potential. However, since for technical reasons, usually all sensors of a series circuit of power semiconductor modules are at identical potentials, the potential difference between the power circuit of a power semiconductor module and the sensor can be higher than within the power circuit itself.
For that reason, two partial tests are conducted to check the isolation strength of a power module with sensor elements or other additional integrated functions. In the first partial test, the sensor is placed at a common high potential with the function circuit, and the base isolation is tested in an ambient environment, whereby the same requirements apply to the sensor and the associated regions of the metal coating that apply to the regions of the functional circuit.
In the second partial test, the sensor is held at ambient potential, and only the regions of the functional circuit are placed at high potential. A potential gradient occurs between the metal coating belonging to the functional circuit and that belonging to the sensor, and this requires appropriate measures to ensure basis isolation. Both partial tests together ensure that the sensor can be operated at any potential between the ambient potential and the maximum admissible isolation voltage of the power module.
The difficulties resulting from these requirements are also discussed in EP 1 111 970, and a method of improving the isolation strength is suggested, but without using the geometrical optimization presented in the present invention.
It is an object of the present invention to provide a power semiconductor module of high isolation strength which overcomes the drawbacks of the related art.
It is another object of the present invention is to increase the isolation strength of the base isolation of power semiconductor modules, whereby these modules maybe additionally provided with metal coatings (at ground potential or at another potential) for sensors and/or drive circuits of the substrate, and to improve the partial discharge characteristics.
It is another object of the present invention to provide a design which equalizes a tangental component of field lines or equipotential lines between a first edge on a first surface of a substrate with a second edge on a second surfaced of the substrate.
The present invention relates to a power semiconductor module with high isolation strength that achieves high isolation strength from a base through selectively positioning a plurality of metal coatings on first and second surfaces and positioning edges of the plurality to beneficially reduce the field strength tangentially to a selected position, especially in a defined critical region directly adjacent a metal coating edge on the first surface. This results in providing regions which beneficially allow field lines to extend without functional detriment. The beneficial selection is is achieved by means of an optimization process in which the tangential components of the field strength beside the first or second metallization edge reach identical values.
According to an embodiment of the present invention, there is provided a power semiconductor module, including connective elements enabling effective operation and at least one semiconductor component, the power semiconductor module comprising: at least one substrate, the substrate having at least a first surface opposite a second surface, a substrate edge on the substrate joining the first and the second surface, at least a first metal coating on a first surface of the substrate, at least a second metal coating on a second surface of the substrate, the second metal coating between the second surface and a facing surface, the first metal coating having a first metal coating edge, the first metal coating edge a distance a from the substrate edge, the second metal coating having a second metal coating edge, at least the first metal coating edge at a high potential to the substrate edge during an operation of the power semiconductor module, the second metal coating edge a distance b from the substrate edge, and the distance a being less than the distance b, whereby field lines existing between the first metal coating and the second metal coating, and through the substrate, during the operation, beneficially extend away from the second surface opposite the first metal coating edge, thereby beneficially reducing a tangential component of a field strength proximate the first metal coating edge, reducing the field strength and field density on the first surface, and increasing an isolation strength of a base isolation of the substrate.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the distance a and the distance b are selected to equalize the tangential component of the field strength proximate the first metal coating edge with another tangential component of the field strength proximate the second metal coating edge.
According to another embodiment of the present invention there is provided a power semiconductor module, further comprising: a difference x defined between the distance a and the distance, b, and the difference x being an absolute value between 0.75 mm and 1.25 mm.
According to another embodiment of the present invention there is provided a power semiconductor module, the substrate is a ceramic selected from the group consisting of aluminum oxide, aluminum nitrite, beryllium oxide and silicone nitrite.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the first metal coating includes a metal selected from the group consisting of copper, aluminum, and silver, and the second metal coating includes a metal selected from the group consisting of copper, aluminum, and silver.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the first metal coating and the second metal coating are metal alloys.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the first metal coating and the second metal coating are a plurality of layers, and the first metal coating and the second metal coating are a plurality of metals in a mixed form.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the first metal coating and the second metal coating are applied on respective first and second surfaces of the substrate by at least one of a Direct Copper Bonding (DCB) process and an Active Metal Brazing (AMB) process.
According to another embodiment of the present invention there is provided, a power semiconductor module, including connective elements enabling effective operation and at least one semiconductor component, the power semiconductor module comprising: at least one substrate, the substrate having at least a first surface opposite a second surface, a substrate edge on the substrate joining the first and the second surface, at least a first metal coating on a first surface of the substrate, at least a second metal coating on a second surface of the substrate, the second metal coating between the second surface and a facing surface, the first metal coating having a first metal coating edge, the first metal coating edge a distance f from the substrate edge, the second metal coating having a second metal coating edge, at least the first metal coating edge at a higher potential to the substrate edge during an operation of the power semiconductor module than at least a third metal coating on the first surface between the first metal edge and the substrate edge, the third metal coating having a third metal coating edge opposite the first metal coating edge and a fourth metal coating edge, the second metal coating edge a distance g from the substrate edge, and the distance f being less than the distance g, whereby field lines existing between the first, the second, and the at least third metal coating, and through the substrate, during the operation, beneficially extend away from the second surface opposite the first metal coating edge, thereby beneficially reducing a tangential component of a field strength proximate the first metal coating edge, reducing the field strength and field density on the first surface, and increasing an isolation strength of a base isolation of the substrate.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the distance f and the distance g are selected to equalize the tangential component of the field strength proximate the first metal coating edge with a tangential component of the field strength proximate the second metal coating edge.
According to another embodiment of the present invention there is provided a power semiconductor module, further comprising: at least fourth metal coating on the second surface of the substrate between the second metal coating and the substrate edge, the at least fourth metal coating having a fifth metal coating edge opposite the second metal coating edge and a sixth metal coating edge, the third metal coating edge and the fifth metal coating edge at similar distances from the substrate edge, a distance c between the first metal coating edge and the third metal coating edge, a distance e between the second metal coating edge and the fifth metal coating edge, and the distance c being less than the distance e, whereby a density of field lines adjacent the first metal coating edge are widened and the field strength on the first surface of the substrate is beneficially reduced.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the distance e is 2.0 mmxc2x10.25 mm and the distance c is 1.0 mmxc2x10.25 mm.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the substrate is a ceramic selected from the group consisting of aluminum oxide, aluminum nitrite, beryllium oxide and silicone nitrite.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the first, the second, the third, and the fourth metal coatings include a metal selected from the group consisting of copper, aluminum, and silver.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the first, the second, the third, and the fourth metal coatings are metal alloys.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the first, the second, the third, and the fourth metal coatings are each a plurality of layers, and the first, the second, the third, and the fourth metal coatings are each a plurality of metals in a mixed form.
According to another embodiment of the present invention there is provided a power semiconductor module, wherein: the first, the second, the third, and the fourth metal coatings are applied on respective first and second surfaces of the substrate by at least one of a Direct Copper Bonding (DCB) process and an Active Metal Brazing (AMB) process.
According to another embodiment of the present invention there is provided a power semiconductor module, further comprising: at least a fifth metal coating on the first surface between the third metal coating and the substrate edge, at least a sixth metal coating on the second surface between the fourth metal coating and the substrate edge, the fifth metal coating having a seventh metal coating edge adjacent the fourth metal coating edge and an eight metal coating edge, the sixth metal coating having a ninth metal coating edge adjacent the sixth metal coating edge and a tenth metal coating edge, a distance h defined between the fourth metal coating edge and the seventh metal coating edge and equivalent to the distance c, a distance I defined between the sixth metal coating edge and the ninth metal coating edge and equivalent to the distance e, and the eighth metal coating edge being nearer to the substrate edge than the seventh metal coating edge, whereby the distances e, I, f, g, and c are selected to equalize tangential components of the field strength proximate the first and the seventh metal coating edge with tangental components of the field strength proximate the second and the ninth metal coating edge, thereby, reducing the field strength and field density on a non-coated region of the first surface, and increasing an isolation strength of a base isolation of the substrate.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.